Activation via the B cell antigen receptor (BCR) by antigen leads to B cell proliferation and immunoglobulin (Ig) secretion. However, engagement of the BCR simultaneously with the Ig receptor on B cells (FcgammaRIIB1), delivers a potent inhibitory signal that prevents B cell proliferation and antibody secretion. Such coligation, which can be mediated by antigen-antibody complexes that can exist in later stages of the immune response, is thought to act as a "feedback suppression" to limit excessive antibody production. FcgammaRIIB1 knockout mice display enhanced antibody and anaphylactic responses and strain-dependent autoimmunity In humans, certain rheumatoid factors, such as an IgM that reacts with the Fc portion of IgG, interferes with FcgammaRIIB1-mediated regulation of antibody production. Thus, understanding the molecular mechanisms of this signaling process has important implications for B cell activation and regulation in physiology and pathology. FcgammaRIIB1 crosslinking with the BCR leads to inhibition of BCR-induced calcium entry into cells, diminution of Ras activation and attenuation of specific transcription factor activation, ultimately leading to inhibition of B cell proliferation. Recently, the inositol phosphatase SHIP has been identified as essential for mediating the inhibitory signaling via FcgammaRIIB1. In vivo, SHIP1dephosphorylates the signaling lipid Ptdlns(3,4,5)P3, which is important for many facets of B cell activation. SHIP1 has also emerged as a critical negative regulator of cytokine and certain growth factor signaling in multiple lineages of hematopoietic cells. Through a combination of functional and biochemical studies, we have identified an essential role for the non-catalytic C-terminal region of SHIP1 in mediating the FcgammaRIIB1-dependent inhibition in cells. However, the specific molecular interactions via the C-terminus that regulates SHIP1 function in vivo are not fully understood. In this proposal, we will attempt to obtain a better molecular understanding of SHIP1 function in FcgammaRIIB1 signaling through genetic, biochemical and functional approaches in SHIP1-deficient cell lines, as well as mice with targeted knock-in mutations. We will also examine interactions of specific signaling proteins with the C-terminal motifs of SHIP1 and determine their significance in FcgammaRIIB1-mediated inhibition. The detailed molecular understanding of FcgammaRIIB1/SHIPl-mediated inhibitory signaling may yield important clues to B cell activation and desensitization, and point to possible future therapeutic interventions in certain autoimmune states.